Skywave Signals Research Articles

Measurement and Analysis of Loran-C Sky Waves throughout the Day

In this study, Loran-C signals were collected throughout the day, and the characteristics of the received signals at different propagation distances were analyzed. Because the signal amplitude is small and difficult to recognize at a long distance and there is mutual interference between stations, a linear average method is used to process the received signal. At locations closer to the receiver, clear observations of the time delay and amplitude variation in the one-hop sky wave can be made by using the ground wave as a reference, which can be applied to studying the characteristics and parameter inversion of the lower ionosphere. When the distance is further, the significant enhancement of the sky-wave signal during the night may lead to decreased accuracy in timing and positioning. When the distance is much larger than the propagation range of the ground wave, only the sky-wave signal can be received, and the signal is more stable than when the distance is closer. During the night, multiple amplitude-comparable multi-path signals may appear in the sky wave, making the identification of the one-hop sky wave more difficult.

The Effect of Random Characteristics of Ionosphere on the Propagation of ELoran Sky Waves

In this study, the effect of random characteristics of the ionosphere on the enhanced Loran (eLoran) sky-wave signal was analyzed. First, the bilinear transform stochastic finite-difference time-domain (S-BT-FDTD) method in layered half-space isotropic plasma is derived, and the correctness of the calculation of the mean value and standard deviation of the field components of the eLoran sky-wave signal is verified. Second, the mean value and standard deviation of the amplitude and time delay of the eLoran sky-wave signal were analyzed by combining the International Reference Ionosphere, pseudo-1-D BT-FDTD method, and Monte Carlo method. The simulation results show that the mean value of the amplitude of the reflection coefficient amplitude (RA) in the eLoran signal bandwidth remains unchanged. The larger the standard deviation of the electron density, the larger is the standard deviation of RA. The standard deviation of RA during the day is greater than that at night. The mean value of the time delay remains unchanged, and the standard deviation of the time delay is related to the way the electron density varies randomly. When the incident angle increases, the RA increases, the standard deviation of RA decreases, the time delay decreases, the standard deviation of the time delay decreases, and the signal becomes more stable.

Crowdsourced Doppler measurements of time standard stations demonstrating ionospheric variability

Abstract. Ionospheric variability produces measurable effects in Doppler shift of HF (high-frequency, 3–30 MHz) skywave signals. These effects are straightforward to measure with low-cost equipment and are conducive to citizen science campaigns. The low-cost Personal Space Weather Station (PSWS) network is a modular network of community-maintained, open-source receivers, which measure Doppler shift in the precise carrier signals of time standard stations. The primary goal of this paper is to explain the types of measurements this instrument can make and some of its use cases, demonstrating its role as the building block for a large-scale ionospheric and HF propagation measurement network which complements existing professional networks. Here, data from the PSWS network are presented for a period of time spanning late 2019 to early 2022. Software tools for the visualization and analysis of this living dataset are also discussed and provided. These tools are robust to data interruptions and to the addition, removal or modification of stations, allowing both short- and long-term visualization at higher density and faster cadence than other methods. These data may be used to supplement observations made with other geospace instruments in event-based analyses, e.g., traveling ionospheric disturbances and solar flares, and to assess the accuracy of the bottomside estimates of ionospheric models by comparing the oblique paths obtained by ionospheric ray tracers with those obtained by these receivers. The data are archived at https://doi.org/10.5281/zenodo.6622111 (Collins, 2022).

Deep Electron Density Depletion Near Sunset Terminator on St. Patrick's Day Storm and Its Impacts on Skywave Propagation

AbstractThe plasma density distribution in the equatorial and low latitude ionosphere can often be severely disturbed during active space weather events that can have paramount impacts on the long‐distance high frequency (HF) radio wave (Skywave) communications through the ionosphere. On the St. Patrick's Day storm of 17 March 2015, a deep depletion of plasma with more than two orders of magnitude was observed over a narrow longitudinal sector near the sunset terminator. The controlled SAMI2 (Sami2 is Another Model of the Ionosphere) simulations indicate that a large equatorial vertical drift around sunset terminator can produce such a deep electron density depletion and strong reinforcement of equatorial ionization anomaly. The impacts of these ionospheric density disturbances on the Skywave communication systems have been investigated using an HF propagation simulator that solves the propagation path of radio waves under given background ionospheric conditions. The results clearly demonstrate that the usable HF spectrum for Skywave communications is reduced by more than 50% over the region of depletion. Further, large areas of skip zones, where the Skywave signals are not receivable, are produced due to low ionospheric densities over this region. This study can have important applications in the planning and operation of Skywave systems during the active space weather periods.

Observations and analysis of the impact of annular eclipse on 10 MHz short-wave signal in Sanya area on June 21, 2020

This paper takes the annular solar eclipse that occurred in China on June 21, 2020, as the background and conducts tests and analysis of shortwave timing sky wave signals. The test group continuously tested the signal field strength data from June 20, 2020, to June 23, 2020. The results show that the shortwave signal field strength on June 21, the day of the annular eclipse, is significantly weaker than that on two other days—before and during the annular eclipse—and there are three obvious jumps in the field strength value. (1) The first jump occurs more than one hour before the annular eclipse, and then, the field strength has an oscillatory attenuation. Between the first jump and the second jump, the overall change trend of the field strength value is an oscillatory decline one. (2) About 25 min before the beginning of the eclipse, the second jump of the field strength occurs; from the second jump to the third jump, that is, before the occurrence of the beginning of the eclipse and the transition phase from the beginning to the middle of the eclipse, the value of the field strength has an oscillatory increase. (3) During the period of 8:00–8:10, the field strength shows a third jump. Although the field strength oscillates after the third jump, it has no obvious increasing or decreasing trend. Based on the results of previous studies on the annular eclipse on that day and the analysis of the theoretical model of radio wave propagation, this paper preliminarily considers that the change in signal field strength is supposed to be caused by the impact of the outbreak of the annular eclipse on the ionosphere. When the annular eclipse occurred, the sunlight reaching the ground is blocked by the Moon, and the ionization degree of electrons in the ionosphere is reduced, resulting in an increase in the penetration factor, and the overall performance of the shortwave signal reaching the ground is weakened.

Insight into skywave theory and breakthrough applications in resource exploration

Skywave refers to the electromagnetic wave reflected or refracted from the ionosphere and propagated in the form of a guided wave between the ionosphere and the Earth's surface. Since the skywave can propagate over large distances, it has been widely used in long-distance communication. This paper explores and demonstrates the feasibility of skywave for deep resource and energy exploration at depths of up to 10 km. Theoretical and technical advancements were accomplished in furthering skywave applications. A new solution method based on Green's function has been developed to study skywave propagation in a fully coupled lithosphere-air-ionosphere full space model. For the first time, the model allows one to study skywave distribution characteristics in the lithosphere containing inhomogeneity such as ore deposits or oil and gas reservoirs. This model also lays a foundation for skywave data processing and interpretation. On a parallel line, we have developed a multi-channel, broadband, low-noise, portable data acquisition system suitable for receiving skywave signals. Using the skywave field excited by a high-power fixed source located in central China, actual field surveys have been carried out in some areas in China including the Biyang depression of Henan Province. The initial results appear encouraging—the interpreted resistivity models prove to be consistent with those of seismic exploration and known geological information, and the exploration cost is only ∼1/4 to 1/10 that of seismic surveys. These initial successful applications of the skywave theory lay a solid foundation for further verification of the new method.

Field-Strength Variations of LF One-Hop Sky Waves Propagation in the Earth–Ionosphere Waveguide at Short Ranges

One-hop sky-wave signals in the Earth–ionosphere waveguide are studied based on the traditional wave-hop theory and the finite-difference time-domain (FDTD) method. Field-strength variations of 100 kHz one-hop sky waves are given under three different waveguide models. The great-circle distance between the transmitter and the receiver is within 200 km. Together with a sky- and ground-wave separation technique in time domain, the narrowband Loran-C signals are employed in the FDTD method. Compared to the results of wave-hop theory, field-strength variations of low-frequency one-hop sky waves are better characterized by the FDTD method as the waves propagate in an irregular Earth–ionosphere waveguide.

Analysis of Signal Reception Mechanism of Sky Wave Long-distance Maritime Communication

Based on the principle of high frequency radio propagation, the signal reception mechanism of sky wave communication over oceans is investigated. Due to long distance signal transmission, the energy loss is inevitable, especially in the space and on the sea surface. Firstly, we establish the space propagation loss model by the ionospheric absorption, the free space propagation characteristics and other extra loss. Referring to the reflection principle of smooth ground and the Kirchhoff approximation, the energy loss models of the calm sea surface and the turbulent sea surface are obtained respectively. Then, through combining the space propagation loss model and the sea surface propagation loss models, we give out a formula of receiving point field strength. According to the signal to noise ratio, we summarize a complete and concise sky wave maritime communication calculation process, through which multi hops number of the receivable sky wave signal can be calculated accurately. The experimental results show the effectiveness.

Feasibility study for reconstructing the spatial‐temporal structure of TIDs from high‐resolution backscatter ionograms

AbstractOver‐the‐horizon radar (OTHR) utilizes the reflective “sky wave” property of the ionosphere for high‐frequency radiowaves to illuminate targets at ranges extending to several thousand kilometers. However, the ionospheric “mirror” is not static but exhibits geographic, diurnal, seasonal, and solar cycle variations. NorthWest Research Associates has developed an ionospheric data assimilation capability called Global Positioning Satellite Ionospheric Inversion (GPSII; pronounced “gypsy”) that allows real‐time modeling of the ionospheric structure for the purpose of accurate coordinate registration (CR; OTHR geolocation). However, the ionosphere is routinely subjected to traveling ionospheric disturbances (TIDs), and the deflection of HF sky wave signals by unmodeled TIDs remains a troubling source of CR errors (tens of kilometers). Traditional OTHR tools for ionospheric sounding (vertical and backscatter ionograms) do not resolve the fine spatial structure associated with TIDs. The collection of backscatter ionograms using the full aperture of the OTHR was recently demonstrated, thus providing enhanced resolution in radar azimuth in comparison with conventional OTHR backscatter soundings that utilize only a fraction of the OTHR receiver array. Leading edges of such backscatter ionograms demonstrate prominent spatial features associated with TIDs. We investigate the feasibility of recovering TID perturbations of ionospheric electron density from high‐resolution backscatter ionograms. We incorporated a model of naturally occurring TIDs into a numerical ray tracing code that allows the generation of synthetic OTHR data. We augmented GPSII to assimilate time series of full‐aperture backscatter ionogram leading edge data. Results of the simulation show that GPSII is able to reproduce the TID structure used to generate the backscatter ionograms reasonably well.

Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud‐to‐ground lightning discharges

AbstractWe analyze broadband electromagnetic measurements of pulse sequences occurring prior to first return strokes of negative cloud‐to‐ground lightning flashes. Signals generated by lightning discharges were recorded close to the thunderstorm by a magnetic field receiver and traveled up to 600 km to three distant electric field receivers. We found that amplitudes of observed preliminary breakdown pulses, as well as amplitudes of the corresponding return strokes, are attenuated approximately by 2 dB/100 km when propagating in the Earth‐ionosphere waveguide over mountainous terrain. Propagation simulations show that there is a significant contribution of the sky wave signals in the waveforms observed beyond 500 km from their source. The estimated peak currents of the largest preliminary breakdown pulses reach over 60 kA. Such current pulses propagating through in‐cloud lightning leader channels in a strong electric field may be able to initiate terrestrial gamma ray flashes.

Time–frequency method to reduce contamination by ionospheric fast frequency modulation for HF skywave radars

The unstable and time-varying property of the ionosphere causes a frequency modulation on the high frequency sky-wave radar echo signals. When the modulation period is of 10 s or smaller, it is called the fast phase-path modulation. This sort of modulation will result in the broadening, the splitting and even the overlapping of the spectra, which makes target detection and sea-state remote sensing hard. Hence, it is necessary to decontaminate the echo signals before the coherent integration process. In this study, the pseudo Wigner-Ville distribution is proposed as a means of estimating the ionospheric frequency modulation. A frequency weighting method is used to obtain the instantaneous frequency (IF) from the time–frequency distribution and an adaptive low-pass filter is introduced to purify the IF. Then, the amplitude difference between the two first-order Bragg peaks is adopted in the weighting process to obtain the phase compensation function. To improve the compensation effect, an optimising process is proposed. The simulation results show that the method can improve the quality of the spectra for the target detection and remote sensing efficiently even in the cases of a high sea-state.

Algorithm of sky-ground-wave signal separation in CDMA system

To solve the problem of the sky-wave interference in radio positioning system operating in CDMA mode, an algorithm of sky-ground-wave separation is provided. Based on the MLE (maximum likelihood estimate), and by estimating the amplitude and the phase of the sky-wave signal, the provided algorithm for separating sky-ground-wave is implemented. The mathematics model used for signal processing is established, and the possible solutions are provided. The structure and signal processing flow implementing the presented algorithm in the receiver are presented. A multi-channels signal searching idea is adopted, some of which process the sky-wave signal, and some of which process the ground-wave signal. Numerical analysis and simulation show that the proposed algorithm has higher accuracy, more rapid processing speed, and simpler implementation for the estimation of the sky-wave signal parameter, and can separate the sky-wave signal and ground-wave signal from the arrival combination signal effectively.

A method for determining intracloud lightning and ionospheric heights from VLF/LF electric field records

The Los Alamos Sferic Array (LASA) recorded VLF/LF electric‐field‐change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential‐times‐of‐arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1‐hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative‐polarity events occurring above ∼15 km and positive‐polarity events occurring below ∼15 km. The negative‐polarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground‐reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

Effect of skywave interference on coverage of radiobeacon DGPS stations

Coastal radiobeacon stations, which have long served marine direction-finding receivers, are increasingly being employed to transmit differential satellite navigation system (DSNS) correction messages to marine users. The range of these 300 kHz data transmissions extends beyond the nominal ranges of the radiobeacons into regions where skywave effects become significant, especially at night. The paper demonstrates by statistical analysis and experimental measurements that skywave-propagated components interact with the wanted groundwaves, causing deep signal fading that can lead to loss of data. Also, in Europe, skywave signals from other radiobeacons on the same, or adjacent, channels cause severe interference. A coverage and performance prediction model that takes these effects into account, together with atmospheric noise and attenuation due to ground losses, is used to illustrate the reduction of usable range caused by skywave propagation. The paper argues that it is essential to allow for skywave factors when predicting the operating areas of beacons and when allocating frequencies within the radiobeacon DSNS service.

LF/MF skywave propagation at daytime

Interference from the daytime skywave signal of one broadcasting station to the groundwave signal of a co-channel station located several hundred kilometers away has been observed in certain parts of the world. The need for a better of skywave propagation at daytime, the Radiocommunications Sector of the International Telecommunications Union (ITU-R, formerly known as the International Radio Consultative Committee, CCIR) adopted Question 215/3 at its 1993 Radio Assembly. This paper is in direct response to this Question. Daytime measurements of more than 30 paths are believed to be skywave and have been studied for detailed information of these paths. The purpose of this paper is twofold: to discuss daytime skywave propagation in a qualitative manner and to present a simple procedure for estimating daytime skywave field strength and interference levels.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurements of direction of arrival of HF sky wave signals by means of a wide aperture antenna array and two super resolution direction finding algorithms

From measurements of the amplitude and phase of ionospherically propagated HF radio signals received at several spaced antennas, the direction of arrival of the signals in both azimuth and elevation can be determined. A series of measurements has been undertaken using a wide aperture antenna array on signals received from several European broadcast stations (in the 9-15 MHz range) at distances of up to about 1400 km, and the directions of arrival estimated by means of two super-resolution algorithms; MUSIC and an iterative null steering method (an approximate maximum likelihood estimator) based on the IMP and DOSE algorithms. A detailed comparison of the performance of the two algorithms has been undertaken, and it is concluded that the null steering algorithm is significantly better than MUSIC at resolving the various propagation modes, particularly for short signal analysis periods.

Novel Techniques for Loran-C Skywave Estimation

It is conventionally assumed that Loran-C receivers process only samples taken before the first skywave components arrive, 35-60 u s after the groundwave. This technique has significant limitations when implemented in receivers of finite bandwidth, which increase the rise times of the pulses and substantially reduce the amplitudes of the groundwave signals at the 30 KS point. For this reason, many current receivers take samples later in the pulse and so suffer skywave errors. This paper examines the consequences of this practice. It then proposes a new class of techniques to allow the onset of skywave signals to be detected so that the sampling point can be adjusted accordingly. Novel algorithms for distinguishing skywaves from the groundwave are presented. Their benefits, and the computing resources they require, are assessed. The paper includes theoretical analyses, simulations, and Monte Carlo experiments to demonstrate the performance of the techniques proposed.

Use of the Wigner–Ville distribution to compensate for ionospheric layer movement in high-frequency sky-wave radar systems

High-frequency (HF) sky-wave radar signals propagate to and from the target by means of the ionosphere. The difficulty in obtaining very narrow beamwidths at HF, and the long ranges involved, mean that the signal backscattered from the ground is often several orders of magnitude greater than the target echo. To resolve the target, coherent signal processing techniques are necessary, detecting the target echo by virtue of the Doppler shift caused by the target&apos;s radial velocity. Unfortunately, movement of the ionospheric layer by which the radar signal has propagated often causes the clutter spectrum and target to spread in the frequency domain, rendering extended coherent integration pointless. The movement of the ionosphere can be regarded as producing frequency modulation of the radar signal, and thus if this modulation can be estimated the radar signal may be corrected for the ionospheric contamination. A technique using the complex argument of the first-order moment of the Wigner–Ville distribution of the filtered radar echo is proposed as a means of estimating the ionospheric frequency contamination, and a correction technique is discussed. Examples of radar data corrected using the technique are shown.

Measurements of the direction of arrival of HF sky wave signals and single site location using a seven element wide aperture (294 m) interferometer array

From measurements of the amplitude and phase of ionospherically propagated HF radio signals received at several spaced antennas, the direction of arrival (DOA) of the signal in both azimuth and elevation can be determined. Furthermore, by combining the DOA with reflection height measurements, the transmitter location can be estimated from a single receiving site. The direction of arrival has been measured for signals received from several European broadcast stations at distances of up to about 1400 km and the position fixes obtained are discussed. The performance of the system was severely affected by multimoded propagation and techniques for recognising periods of single moded propagation, when accurate measurements are to be expected, are described. Good position fixes were obtained when measurements were restricted to these periods provided that accurate height information was available. A period of data is also discussed for which the signal was found to be propagated predominantly by a 2-hop F-region reflection. A wide spread of fixes for the ground reflection point was found and this is assumed to be caused by the roughness of the ground surface at the reflection point.

HF multipath dispersion measurements using cepstral signal processing

A technique for measuring ionospheric echo arrival times and strengths is described. The technique uses ordinary HF‐SSB transceivers and an easy to generate probe signal. It is based on the cepstral signal processing tool for deconvolution. Identification of ionospheric echoes in a received sky wave signal was viewed as a deconvolution problem, and the cepstral transformation was used to obtain information about the multipath profile of the channel. Field tests were carried out where multipath characteristics of the ionospheric channel was measured using this technique.